Morbidity in schistosome-infected individuals is caused mainly by the immune response of the host to schistosome eggs deposited in the tissues. In chronic schistosomiasis, fibrotic sequelae to the inflammatory reaction to the eggs are responsible for most clinical disease. Hepatic fibrosis and the granulomatous response to eggs of schistosome species pathogenic for man are studied in mice in relation to parasitologic parameters of infection. Cytokines play an important role in the genesis and regulation of the size of circumoval granulomas and in the fibrosis associated with them. Treatment of mice with antibodies to IL-4 prior to egg laying markedly decreased hepatic fibrosis in S. mansoni and S. Japonicum-infected mice while having treatment of naive mice injected intravenously with schistosome eggs inhibited the development of pulmonary granulomas. Our results suggest that Th2-type cell responses are important for the fibrotic responses to schistosome eggs and, by analogy, perhaps for other hypersensitivity reactions in which "delayed hypersensitivity: is important.